Substrates for printed circuit boards are widely used, substrates including metal layers composed of, for example, metals stacked on surfaces of insulating resin films composed of, for example, resins and being configured to provide printed circuit boards by etching metal layers to form conductive patterns.
There is a need for a substrate for a printed circuit board, the substrate having a high peel strength between a resin film and a metal layer such that when a folding force is applied to a printed circuit board formed from the substrate for a printed circuit board, the metal layer is not peeled from the resin film.
A trend toward smaller and higher-performance electronic devices has recently required higher-density printed circuit boards. In high-density printed circuit boards, finer conductive patterns are more liable to be peeled from resin films. Thus, there is a need for a substrate for a printed circuit board, the substrate being capable of providing a fine conductive pattern and having a good adhesion between a metal layer and a resin film as a substrate that meets the requirement for higher density for a printed circuit board.
To address the requirements, a technique is known in which the adhesion between a metal layer and a resin film is increased by forming a thin copper layer using, for example, a sputtering method on a surface of the resin film and then forming a thick copper layer thereon using an electroplating method. However, in the case where a metal layer is directly deposited on a resin film, it is known that main metal atoms of the metal layer diffuse with time in the resin film to decrease the adhesion between the metal layer and the resin film.
A technique is reported in which a thin chromium layer is deposited by sputtering on a surface of copper foil to be bonded to a resin film, and the resulting copper foil is bonded to the resin film by thermocompression bonding (see Japanese Unexamined Patent Application Publication No. 2000-340911). The presence of a thin layer of a metal different from the main metal of a metal layer at the interface between the metal layer and a resin film provides the effect of hindering the migration of the main metal of the metal layer to the resin film to inhibit a decrease in the adhesion between the metal layer and the resin film due to the diffusion of the main metal atoms of the metal layer in the resin film.